1. Technical Field
This disclosure relates generally to systems and methods for detecting, diagnosing and treating cardiovascular disease in a medical patient using cardiac rhythm management devices and methods that use novel digital electrode technology.
2. Description of the Related Art
The optimum management of patients with chronic diseases requires that therapy be adjusted in response to changes in the patient's condition. Ideally, these changes are measured by daily patient self-monitoring prior to the development of symptoms. Self-monitoring and self-administration of therapy forms a closed therapeutic loop, creating a dynamic management system for maintaining homeostasis. Such a system can, in the short term, benefit day-to-day symptoms and quality-of-life, and in the long term, prevent progressive deterioration and complications.
In some cases, timely administration of a single dose of a therapy can prevent serious acute changes in the patient's condition. One example of such a short-term disease management strategy is commonly used in patients with asthma. The patient acutely self-administers an inhaled bronchodilator when daily readings from a hand-held spirometer or flowmeter exceed a normal range. This has been effective for preventing or aborting acute asthmatic attacks that could lead to hospitalization or death
In another chronic disease, diabetes mellitus, current self-management strategies impact both the short and long term sequelae of the illness. Diabetic patients self-monitor blood glucose levels from one to three times daily and correspondingly adjust their self-administered injectable insulin or oral hypoglycemic medications according to their physician's prescription (known as a “sliding scale”). More “brittle” patients, usually those with juvenile-onset diabetes, may require more frequent monitoring (e.g., 4 to 6 times daily), and the readings may be used to adjust an external insulin pump to more precisely control glucose homeostasis. These frequent “parameter-driven” changes in diabetes management prevent hospitalization due to symptoms caused by under-treatment (e.g., hyperglycemia with increased hunger, thirst, urination, blurred vision), and over-treatment (e.g., hypoglycemia with sweating, palpitations, and weakness). Moreover, these aggressive management strategies have been shown to prevent or delay the onset of long-term complications, including blindness, kidney failure, and cardiovascular disease.
There are approximately 60 million people in the U.S. with risk factors for developing chronic cardiovascular diseases, including high blood pressure, diabetes, coronary artery disease, valvular heart disease, congenital heart disease, cardiomyopathy, and other disorders. Another 10 million patients have already suffered quantifiable structural heart damage but are presently asymptomatic. Still yet, there are 5 million patients with symptoms relating to underlying heart damage defining a clinical condition known as congestive heart failure (CHF). Although survival rates have improved, the mortality associated with CHF remains worse than many common cancers. The number of CHF patients is expected to grow to 10 million within the coming decade as the population ages and more people with damaged hearts are surviving.
CHF is a condition in which a patient's heart works less efficiently than it should, and a condition in which the heart fails to supply the body sufficiently with the oxygen-rich blood it requires, either during exercise or at rest. To compensate for this condition and to maintain blood flow (cardiac output), the body retains sodium and water such that there is a build-up of fluid hydrostatic pressure in the pulmonary blood vessels that drain the lungs. As this hydrostatic pressure overwhelms oncotic pressure and lymph flow, fluid transudates from the pulmonary veins into the pulmonary interstitial spaces, and eventually into the alveolar air spaces. This complication of CHF is called pulmonary edema, which can cause shortness of breath, hypoxemia, acidosis, respiratory arrest, and death. Although CHF is a chronic condition, the disease often requires acute hospital care. Patients are commonly admitted for acute pulmonary congestion accompanied by serious or severe shortness of breath. Acute care for congestive heart failure accounts for the use of more hospital days than any other cardiac diagnosis, and consumes in excess of 20 billion dollars in the United States annually.
Cardiac rhythm management devices such as pacemakers, are an important tool in the treatment of cardiovascular diseases. Typically, an implantable pacemaker uses a minimum of two electrodes to stimulate tissue. At least one of these electrodes is in contact with the heart tissue to be stimulated, and is called a pacing electrode. The required second electrode need not be in contact with tissue being stimulated, in which case it is called an “indifferent” electrode. The indifferent electrode does not even have to be in the heart. Cardiac pacemakers in commercial use today all have the same basic configuration in which stimulating electrical pulses are produced by a pulse generator located outside the heart, typically in a subcutaneous pocket in the upper chest near one shoulder. The stimulating electrical pulses are applied to the electrodes via one or more electrical conductors within an insulated flexible cable, or “lead”, which is connected at its proximal end to the pulse generator. The distal end of the lead is placed within the heart at a desired pacing location, for example in the apex of the right ventricle. Some pacemaker leads, called “unipolar” leads, have only a pacing electrode, typically at the distal end of the lead. In this case, the required indifferent electrode may be provided by the metallic housing of the generator, or conceivably could be located on another lead. Commonly, unipolar pacemaker leads have a single conductor connecting the generator to a single pacing electrode located at its distal end. Bipolar pacemaker leads have two conductors, one connected to a pacing electrode located at or near the distal end of the lead, the other connected to an “indifferent” electrode, usually configured as a ring electrode, located on the lead some distance proximal to its distal end.